This application is based upon and claims benefit of priority of Japanese Patent Application No. 2002-377259 filed on Dec. 26, 2002, the content of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a pressure control valve used in a fuel injector for supplying fuel to an internal combustion engine.
2. Description of Related Art
In a common rail type fuel injection system, pressurized fuel contained in a common rail is supplied to an internal combustion engine through a fuel injector installed in each engine cylinder. The pressurized fuel introduced into the injector is injected into the engine by driving a nozzle needle in a controlled manner. A control chamber into which the pressurized fuel is introduced is provided behind the nozzle needle. The nozzle needle of the injector is controlled by changing fuel pressure in the control chamber. The pressure in the control chamber is in turn controlled by a pressure control valve. More particularly, when the fuel pressure in the control chamber decreases, the nozzle needle is lifted from its seat and the fuel is injected into the engine. When the pressure in the control chamber increases, the nozzle needle sits on its seat, and thereby fuel injection is terminated. The pressure control valve is driven by an actuator such as a piezoelectric actuator.
An example of this kind of injector is disclosed in JP-A-2001-82295. The injector disclosed therein includes a two-way valve that has a control piston for selectively opening an inlet port or an outlet port, both ports communicating with a control chamber. A guiding member for compensating a force applied to the piston is provided at one end of the piston, and a centering member for correctly positioning the piston relative to a valve seat is provided at the other end. When the control piston is lifted from the valve seat, the control chamber communicates with a drain passage, and fuel flows out from the control chamber. When the communication between the control chamber and the drain passage is interrupted, the fuel pressure in the control chamber is increased by pressurized fuel introduced into the control chamber.
Another example of this kind of injector is disclosed in JP-A-2000-130614. The injector disclosed therein has a three-way valve for controlling pressure in a control chamber. The three-way valve has a valve body that is able to sit selectively on a first seat communicating with a drain passage or a second seat communicating with a common rail. By controlling the valve body, the control chamber communicates with either a drain port or a high pressure port. During a period in which fuel is injected, the control chamber communicates with the drain port while closing the high pressure port. In this manner, an amount of fuel flowing out to the drain passage is restricted.
It has been found out, however, that some problems are involved in the conventional pressure control valves, especially in the three-way valve. Namely, operation of the control valve becomes unstable sometimes, and/or energy loss in an actuator driving the control valve becomes high. Further, there is a possibility that durability of the control valve is adversely affected by abrasion wear due to foreign particles contained in fuel.
The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide an improved pressure control valve for use in a fuel injector. More particularly, an object of the present invention is to improve operational stability and durability of the pressure control valve. Another object of the present invention is to provide a fuel injector in which the improved pressure control valve is used.
A fuel injector for supplying high pressure fuel to an engine cylinder includes a nozzle needle and a pressure control valve that controls a back pressure applied to the nozzle needle. The pressure control valve includes a housing having a cylinder bore and a valve chamber, and a valve needle having a valve body and a piston portion. The valve body is disposed in the valve chamber, and the piston portion is slidably disposed in the cylinder bore. The valve needle is driven by an actuator such as stacked piezoelectric elements (a piezo stack). The valve body of the valve needle selectively closes a drain port or a high pressure port of the valve chamber, thereby controlling a fuel pressure in a control chamber in which a back pressure for driving the nozzle needle is developed.
The drain port is closed by closing a drain seat with a first surface formed on the valve body, and the high pressure port is closed by closing a high pressure seat with a second surface formed on the valve body. The first surface of the valve body is made flat and the drain seat is also made flat. The second surface of the valve body is sloped so that it contacts a flat surface of the high pressure seat. Diameter D1 of the drain seat, diameter D2 of the high pressure seat and diameter D3 of the piston portion of the valve body are made to satisfy the formula: D1xe2x89xa7D2xe2x89xa7D3. Preferably, differences among these diameters are made small. Operation of the fuel injector can be made smooth by designing the fuel pressure control valve in this manner. Also, energy loss in the piezo stack for driving the pressure control valve can be made small.
The first surface of the valve body may be made slanted so that the diameter of the drain seat becomes stably constant. The first surface may be composed of two surfaces slanting in respective directions, forming a peak position connecting two surfaces. The peak position contacts the drain seat when the valve needle is driven to a position for closing the drain port. A slanting angle of the surface positioned outside is so made that foreign particles contained in fuel is prevented from entering the valve chamber. The slanting angle may be made, e.g., in a range from 0.5xc2x0 to 10xc2x0. The high pressure seat surface may be sloped so that the outer periphery of the second surface of the valve body contacts the high pressure seat when the high pressure port is closed. In this manner, operation of the valve needle is further stabilized. A tip portion of the needle valve contacting a valve piston driven by the piezo stack may be made a spherical surface to further improve the smooth operation.
According to the present invention, the operation of the fuel injector is stabilized, energy loss in the piezo stack is reduced, and durability of the pressure control valve is improved. Other objects and features of the present invention will become more readily apparent from a better understanding of the preferred embodiments described below with reference to the following drawings.